1. Field of the Invention
The present invention relates to a system for transmitting a signal to a remote device and implementing a respective command, and more particularly, wherein the system features a friend/foe screening technique which provides a signal which is resistant to copying and regeneration.
2. Description of the Related Art
The present invention relates to techniques to prevent duplication of a signal utilized to communicate an implementation command to provide remote vehicle access as well as to remotely control various functions on-board a vehicle. Vehicle access codes used to accomplish such communication are susceptible to unauthorized recordation and duplication. That is, the access code of a transmitter can be duplicated by unauthorized individuals permitting unauthorized access to the vehicle. Such is especially prevalent in radio frequency remote vehicle access devices as recordation can be made with conventional magnetic tape recording devices which may then readily reproduce the signal as desired permitting unauthorized access.
An approach to prevent such duplication is to implement a rolling code. In such a system, the transmitter and receiver each advance to identical new codes each time the unit is used. Therefore, if a code is recorded by an unauthorized individual, the next time the system is used, the receiver has changed to a new code ignoring the sequentially prior code. The increased security provided by use of a rolling code comes at the cost of some inconvenience. That is, the rolling code in the transmitter and the receiver may not stay synchronized, as the transmitter may be actuated beyond the range of the receiver resulting in an increment or roll of the code stored in the transmitter without a corresponding roll of the receiver code. In this case, if the transmitter is within the look ahead range of the receiver, the receiver code will be advanced until a match occurs. However, if the transmitter is beyond the look ahead range, user action will be required. Therein lies the inconvenience as complicated techniques must be utilized to resynchronize the transmitter and receiver pair. Therefore, there is a need for a remote vehicle access system that does not require resynchronization.